Update actions

[mbruns91]

Refactors some of the workflows in this repository's actions.
A fist step was done in #426 , but I think it's cleaner to separate these activities. The fist commit just takes the changes to testing.yml done over there (@niklassiemer) to this PR.

We want to

• build images defined in this repository on seperate runners (avoids running out of disk-space)
• do some generalizations to reduce redundant sections in the workflow definitions.

I'll add things to this list, as I'm doing the changes.